A BAW resonator features a piezoelectric layer, which is arranged between two metal layers (electrodes). Instead of only one piezoelectric layer, a layer sequence can also be used. The layers are deposited on a substrate one after the other and structured to form resonators, which are connected to each other electrically and which together can implement, e.g., a filter circuit. The resonator surface defined by the electrodes or their overlapping area is also called an active region. The thickness of the piezoelectric layer of a BAW resonator, whose resonance frequency be in the frequency range between 0.1-10 GHz, equals at most approximately 0.1-10 μm.
When an electric field is applied to the electrodes of the BAW resonator perpendicular to the layer arrangement, mechanical stress (expansion or compression of the material) is released in the piezoelectric layer of the BAW resonator through the deflection of atoms in the field direction. The deflection of the atoms is mainly in the perpendicular direction (for a c-axis perpendicular to the piezoelectric layer).
A BAW resonator can be provided with an acoustic reflector, which is arranged between a carrier substrate and the BAW resonator. The acoustic reflector is composed of alternating layers of a high and low acoustic impedance, whose layer thicknesses equal approximately one-quarter wavelength of the acoustic fundamental mode (relative to the propagation velocity of the acoustic wave in the appropriate material). The acoustic reflector therefore provides one or more boundary surfaces, which reflect the acoustic wave back into the resonator at the resonance frequency and prevent the emission of the wave in the direction of the carrier substrate.
The thickness of the piezoelectric layer defines the limit frequency of the BAW resonator. The limit frequency is the resonance frequency of the fundamental mode, which is the first harmonic of the vertical longitudinal bulk acoustic wave. The propagation time of the fundamental mode (first harmonic of the vertical longitudinal wave) in the piezoelectric layer of thickness d equals 2 d/vL (vL=propagation velocity of the longitudinal acoustic wave). The frequency of the fundamental mode is then fL1≈vL/2 d.
FBAR resonators can be used for producing passband high-frequency filters and can be used, e.g., in mobile communications terminals. These resonators are also used in frequency-defining elements in oscillators and for sensors.
FBAR filters are being used increasingly in applications in which the use of surface acoustic wave filers (SAW filters) is not possible due to the technical limitations of fabrication. While for SAW filters, the filter structure must be generated by a two-dimensional lithographic structure, for FBAR filters, just the layer thickness of the layers that are used determine the frequency. These can be controlled with considerably more precision and ease than a 2-D structure. While for SAW filters, a technologically feasible upper limit is currently at ca. 5 GHz, for FBAR technology, currently an upper limit at ca. 10 GHz is seen. There exist, however, filter applications between 10 and 12 GHz, for example, for satellite receivers. This frequency range is currently covered by filters based on DRO technology, which uses dielectric resonators. A disadvantage of this technique is that it generates high costs.
FBAR filters and resonators for the range above 10 GHz have already become known, but these are not suitable for mass production due to the expensive technology and are still found in the laboratory sector.